Following the recent spread of multimedia services and expansion of usage of ICT (Information and Communication Technology) services, Internet traffic flowing through a backbone network continues to increase year by year. Attention has been recently paid to a digital coherent technique as a next-generation optical communication technique for driving continuously increasing traffic (see NPL 1, for example). Introduction of the digital coherent technique makes it possible to correct distortion in a transmission path by digital signal processing, and thus makes it possible to perform high-precision dispersion compensation in a wide range. Furthermore, use of the digital coherent technique enables processing such as phase estimation and polarization split to be performed by digital signal processing, and techniques such as multilevel modulation, polarization multiplexing, etc. which have been difficult to be implemented have been widely used.